Bunker 32.

The USA uncovered something in a cave deep underground using thermal radar. They thought it was a russian bunker, They dug down and built a bunker right outside of the cavern, then drilled inside and began exploring it. All that was heard was radio chatter and screams, violent screams. They shut the vault door before anything could escape, 49 men died that day. 30 years later, Some old personel and civilians discovered the site and explored it, accedentally triggering the door. It escaped. It killed everyone. You and the last of your team hid in the bathroom and survived the attack, Everyone else died horrifically.